The present invention relates broadly to delay lines, and in particular, to a programmable, variable length, high speed, digital delay line apparatus.
In the prior art, the function of electrically controlling a variable delay line was usually accomplished through the use of a variable inductance or a variable capacitance in the delay line. By varying the electrical potential applied to the inductance or capacitance, the amount of delay imposed upon the input signal in the delay line could be increased or decreased as desired. The disadvantage of this technique, however, was that the increment of delay which could be imposed on the input signal as a practical matter was relatively a small fraction of the total delay available in the delay line. A further disadvantage resulting from the use of these techniques was that by varying the inductance or capacitance in the delay circuit, the characteristic impedence of the entire line through which the input signal traveled is changed with the result that it becomes difficult to obtain a final signal with the proper wave form characteristics. Methods to automatically reterminate the line as the characteristic impedance is varied, are complex and expensive. The previous method also suffers from a tendency to couple the control signal into the output signal, thereby corrupting its wave form as the delay is varied.